1. Field of the Invention
The present invention relates to an optical disk device for eliminating an offset of an actuator, and an offset eliminating method used thereby
2. Description of the Related Art
Writing or reading by an accurate tracing of a track in an optical disk requires tracking control by a beam spot which can accurately trace the track. Accordingly, an optical head mounted on a carriage is provided with a tracking actuator which moves the beam spot in a radial direction within a predetermined range of the track width in the carriage stationary state. The tracking actuator includes a magnetic field structure, as a stationary member, and a rotation member provided with a tracking coil. The beam spot from an object lens is moved perpendicular to the track of the optical disk by providing an object lens of a head optical system at the rotation member side, and by rotating the rotation member by supplying an electric power to the tracking coil.
Usually, tracking control is accomplished by feedback control of electric current through the tracking coil. A tracking detection signal is held at zero, in accordance with a tracking error detection signal obtained as a difference of an output obtained by making a light reflected from the disk incident on a two-divided photodiode, through the head optical system.
Further, a position detector using a two-divided photodiode for detecting a neutral position of the rotation member of the tracking actuator, i.e., a position at which the rotation is stopped when the electric power supplied to the coil is cut, is provided. The position signal in the track direction (radial direction) is detected as a difference of the two incident lights. The track position detection signal is used for control of the holding of the head carriage, which is controlled by a voice coil motor, and holds the rotation member of the actuator at the neutral position by supplying a drive current to the voice coil motor so as to hold the track direction position detection signal at zero.
As a result, the actuator is controlled by a double servocontrol wherein the actuator is controlled by feedback control to the tracking coil receiving the tracking error signal and to the voice-coil motor receiving the track direction position signal.
The rotation member of the tracking actuator is provided with a spring member for returning the rotation member to the neutral position when the electric current supplied to the coil is cut off. The balance of the position of the rotation member when held by the spring member does not always coincide with the neutral position whereat the detection signal of the position detector is zero.
If a difference occurs between the balance position of the spring member and the zero signal position of the position detector an offset in the tracking error signal due to a spring force is generated when the tracking control by the double servocontrol is executed. If this offset due to the spring force exists, when the tracking control is turned OFF and the track actuator is driven by the velocity control at the track jump, an undershoot or overshoot occurs at the end of the track jump. Accordingly, preferably the control has a response characteristic at the track jump that is not adversely affected because the offset due to the spring force is suppressed.